The present invention relates to a method of the preparation of Fe-Cu based sintered alloys which undergo less dimensional changes during sintering and are thus of a very high density as compared with the conventional alloys of the same type, by suppressing the increase in dimension of the compacts which are mainly composed of iron and copper powders, said increase in dimension or copper growth taking place during sintering.
Among the iron-based sintering materials, the most available alloy element is copper, and Fe-Cu based materials constitute a basic component of a variety of sintered materials.
However, the Fe-Cu based materials have a technically troublesome feature in that a substantial growth therein takes place during sintering, which is referred to as the copper growth phenomenon. In other words, the sintered objects or parts now commercially available are prepared by compression molding of powders to powder compacts having the same dimension as that of the desired objects by means of a die that is dimensioned so as to allow for a dimensional change occurring during sintering, by sintering of the powder compacts, and subjecting the sintered compacts to post-treatments such as sizing or coining as the case may be. Accordingly, a too large or excessive dimensional change leads to a larger variation in the dimension of the compacts. As a result, it is very difficult to obtain parts having the required dimensional accuracy even by sizing.
As a consequence of such a growth during sintering, the density of the compacts under high pressures drop to such a degree that they are not used for mechanical parts.
For these reasons, the sintered material designed for use in or as mechanical parts should have an allowable dimensional change upon sintering of no more than 0.4% from various standpoints including an economical one, said percentage being calculated relative to the dimension of a die used.
Various attempts have so far been made to suppress the copper growth phenomenon. For instance, it has been proposed to attain such suppression by the addition of carbon, phosphorus, nickel or the like; however, no additive giving satisfactory results has yet been found. The addition of carbon has an effect to some extent in the case where the copper content is low. However, at a copper content of no less than 5%, it is impossible to attain a desired dimensional change. This is also true of the case where use is made of phosphorus. The use of phosphorus or carbon in a larger amount causes the sintered compact to be made harder, thus encountering difficulties in sizing upon sintering. Nickel is effective if used in a larger amount; however, it is expensive and unprofitable.
The results obtained by use of other elements are summarized en bloc in Table 1 set forth hereinafter.
Furthermore, it has been proposed to prevent the copper growth by low-temperature sintering. But this is also unpractical since the copper growth already begins at a temperature of 910.degree. C. which is the alpha to beta transformation temperature of pure iron.